GENEVA, Oct 6 (AFP) - How well you sleep could depend on who your parents were, researchers at Switzerland's Lausanne University reported on Thursday after identifying for the first time a "normal sleep" gene among mice.

"The first lesson of our research is that the quality of sleep is genetically determined," Mehdi Tafti, a professor at the university's Centre for Integrative Genomics, told AFP.

The scientists used an electroencephalograph to study low-frequency brain waves among mice.

These so-called delta waves, the result of neuron activity, are telltales of the depth and quality of sleep.

"We realised that some mice slept less well because they lacked delta activity," Tafti said. "When we compared their genes with those of other mice, we localised the gene responsible for this difference."

The finger points at a gene that encodes cell receptor for retinoic acid.

Vitamin A, found in eggs, carrots and other orange or yellow vegetables, is known to act on this receptor.

Scientists already knew that vitamin A plays an important role in the brain, in respect of sight but also diseases such as schizophrenia, Alzheimer's and Parkinson's -- diseases that are also accompanied by a lack of delta activity.

Research on the mice found too much vitamin A was bad for sleep, although the scientists are unsure what the results of a deficiency of the vitamin would be and what the ideal intake is. Too much vitamin A can damage health, especially among pregnant women.

In humans, delta activity diminishes with age, which is why children sleep deeply and awake refreshed, while old people usually sleep less well.

The study appears on Friday in the US weekly journal Science.

Previous research has identified genes that can cause sleeplessness, but this is the first to identify a gene linked with normal sleep.